[Pardon me addressing the OP as “you”; I think it is all going to sound rather contrived otherwise.]
[I suggest searching “terraforming” on this site. I found this:
[I am taking it that you want the colonists to settle near the coast.]
They will probably settle where they arrive. Conversely, I take it they came in from space, so they (presumably) could and would have chosen where to land/settle. (Conversely again, having a very good idea of all the pro’s and con’s tends towards high suspension of disbelief.)
Human beings have been short-sighted in their built-infrastructure planning for the whole of human existence. (Examples: • building on farmable land and • putting traffic lights at/under freeway overpasses.) I think that the reason is that planning well takes so much additional time and work. Conversely, this particular case is (presumably) very thoroughly planned (“money no object”).
I take it that the oceans and rivers are H2O (and that they were able to establish this). A quick Google search reveals that ocean salt comes from rocks, so the ocean will be salty.
The governing factor is that (presumably) the spaceship must have been able to support all the people on board for decades — meaning that it is a functioning closed system. (The spaceship would have been designed with a power source with extra capacity, for when they came to setting themselves up on their new planet.) It is tempting to think that they would race out of the spaceship and start madly working on surviving on the new planet. I am sure that they would work hard on making the planet support (their) life, but there is no danger nor deadline involved.
The point is that the absolutely crucial thing is landing the spaceship somewhere where it will be safe and useful, to the very highest possible level of certainty. It would remain their home for months to years.
Other people have suggested reasons for setting up on the coast. I would like to suggest that landing the spaceship on or very near the beach is a very reliable way of rusting it away in literally a year or two, or a decade or two, depending on the distance. Near the coast is much more sensible.
(One pivotal issue is that of whether or not the planet’s air is breathable, or whether they have to use space suits. I take it that, in this case, it is. I think it would be foolhardy, nonetheless, to allow any of the planet’s air into the spaceship, nor to breathe any of it, until it has been thoroughly checked. This is a short-term issue, unless you want the story to be that the air, or the environment generally, is unsafe.)
I am thinking that they will have a long-term plan to “multiply and fill the earth”, and that this would govern their priorities. I am no expert, but I am fairly confident that the main first thing would be to get farming going. They would certainly use water from the planet, not from the spaceship, for this farming.
Arable soil is not just dirt; Earth dirt [that will support plants] is/supports a complex ecosystem. If this planet is lifeless, the soil is going to be lifeless. (Actually, there might not be soil; you would have to ask someone else. [I would guess, from “The Martian”, that any dust can be soil.]) Either way, there is masses of work involved in getting farming going.
Normally, a flood plain is the best place for crops. Under these artificial conditions, it might not be; I do not know. Either way, I would expect them to look for a large flattish area near water (a large lake or river)… with a secure, flat, sheltered spot nearby for the spaceship.
Short- to medium-term, they want mineral resources nearby.
I do agree that being near the ocean seems like a good idea, but I would consider that a long-term issue. (They would want to seed the ocean with fish, I would think. Unfortunately, the smallest fish eat something other than fish, so that is a long-term project.) Finding a fully ideal spot would be radically unlikely. (As I said) I think that their initial site would be governed… medium- to large-scale, by available minerals, and small-scale by setting up farming.
I can not see them building a desalination plant. They might well have one on the spaceship. Nonetheless, I can not think of a compelling reason for them to have to set up near the ocean and run a desalination plant, short of there being no rivers. (Some rivers on Earth are unreliable in some years, but I think there are some that are reliable.)
I suppose the conclusion is that landing somewhat near the coast is desirable long-term — notably for access to [the] other minerals (and long-term they might set up other colonies with different plant ecosystems, and thus resources) — and that it is no stretch for your story to supply a suitable spot that is near the coast.